Here is an excellent resource for nearly all of your questions: http://idioms.thefreedictionary.com/I+wouldn%27t+put+it+past I wouldn't put it past somebody = something that you say when you think that it is possible that someone might do something wrong or unpleasant or exceptional. 'Do you really think he'd go off with another woman?' 'I wouldn't put it past him.' (often + to do sth) I wouldn't put it past Lorna to deny all knowledge of this plan. Technically, it refers to putting the idea past or beyond someone. The idea of doing this is beyond him = He would never do it. I wouldn't put the idea past/beyond him = I don't think he would never do it. Figure it out.

It's the sort of thing I expect you to do.

"I wouldn't put it past you" means the person saying to other has no doubt in their mind that other was capable of doing what was being talked about.